A sheet including a polyvinyl acetal and a plasticizer has excellent adhesion to glass, transparency, mechanical strength, and flexibility, and therefore is widely used as an interlayer film for laminated glass.
In an interlayer film for laminated glass, having a high water content, bleeding (oozing) of a plasticizer may occur. In addition, in a case where the interlayer film is stored in a warehouse, etc., when the outside temperature is as high as 25° C. to 40° C., in summer, etc., the interlayer film may be deformed, or blocking between the interlayer films may easily occur at such a high temperature. Usually, the interlayer film is stored in a warehouse, etc. adjusted to a low temperature and a low humidity by an air-conditioning equipment, etc. so as not to cause bleeding of the plasticizer, deformation, or blocking.
However, when the interlayer film for laminated glass is stored, the inside of a warehouse may have a high temperature and a high humidity, for example, due to failure of an air-conditioning equipment, and the water content of the interlayer film for laminated glass may be significantly high. When the interlayer film for laminated glass having a high water content is left as it is, water or a plasticizer may cause phase separation in the interlayer film, or the plasticizer may bleed from a surface of the interlayer film to reduce transparency or mechanical characteristics of the interlayer film for laminated glass.
By the way, in recent years, as an interlayer film for laminated glass having sound insulation, a multilayer interlayer film for laminated glass has been examined. As the sound insulating interlayer film for laminated glass, a sound insulating multilayer interlayer film for laminated glass obtained by laminating a layer having a low content of a plasticizer for exhibiting mechanical strength or adhesion to glass and a layer having a high content of the plasticizer for exhibiting sound insulation, is used generally (refer to Patent Literatures 1 and 2).
When the multilayer interlayer film for laminated glass is stored for a long time while having a high water content, the water content becomes significantly high. Water or a plasticizer may cause phase separation in the interlayer film, or the plasticizer may bleed from a surface of the interlayer film. Therefore, it is necessary to dry such a multilayer interlayer film for laminated glass having a high water content in an atmosphere of a low temperature and a low humidity. However, when the multilayer interlayer film for laminated glass having a high water content is dried rapidly in the atmosphere of a low temperature and a low humidity, water or the plasticizer may cause phase separation from a polyvinyl acetal in the interlayer film to make the interlayer film opaque, or water or the plasticizer may bleed between layers of the interlayer film to cause interlayer peeling or reduction in transparency. In order to avoid these problems, drying may be performed while the temperature and the humidity are lowered gradually. However, there is a problem that it takes a long time for drying.
The interlayer film for laminated glass generally includes a plasticizer. The plasticizer may be extracted with water attached to a portion where the interlayer film is exposed at an end of the laminated glass, the glass may be peeled from the interlayer film, or air bubbles may be generated to damage the appearance of the laminated glass. In addition, in some types of plasticizers, a depressurizing step may be performed when laminated glass is manufactured, or a plasticizer may volatilize from an end of the laminated glass after use for a long time.